In the standard of Third Generation (3G) Wideband CDMA (WCDMA) based on Code Division Multiple Access (CDMA), as all subscribers in the same cell and all subscribers in the adjacent cells share the identical frequency band at the same time, there is interference among the subscribers, which restricts system capacity and data transmission rate. To increase the system capacity, manifold diversity methods can be adopted, such as the technology of multi-path diversity, space diversity and antenna diversity, etc. In the diversity-applied system technology, for one piece of information there are more than one independent copies of different formats. After being received by a receiver, these independent copies are specially processed according to Maximum Likelihood (ML) Principle and by taking good use of information redundancy. By doing this, the bit rate error of information transmission is greatly reduced and transmission power of wireless data is decreased, consequently the interference among subscribers being reduced. It shows that system capacity can be effectively improved by using diversity technology.
In the downlink channel of WCDMA system, to implement high-capacity of subscriber number and data transmission with high speed, manifold transmit diversity technologies are also to be applied, and space-time transmit diversity (STTD) is one of those technologies, the content of which being: the information to be transmitted being simply space-time encoded, the space-time encoded signal being divided into two routes which are respectively sent to two independent transmitting channels for transmission according to equal power principle; accordingly, the receiving end respectively receiving the signals from two independent transmitting channels and processing the received signals according to Maximum Likelihood (ML) Principle, as is shown in FIG. 1. In FIG. 1, sign 100 denotes the information to be transmitted, in which S1 and S2 are two transmitting symbols in the same space-time encoding block; sign 101 is the space-time encoding module; sign 106 is the transmitting antenna array, indicating that two dependent transmitting channels perform transmission through antenna Ant1 and Ant2 respectively; Rec is the receiver at the receiving end based on Maximum Likelihood (ML) Principle.
At the receiving end, the received signals of the same space-time encoding module is expressed as follows:
                    {                                                                              r                  1                                =                                                                            h                      1                                        ⁢                                          S                      1                                                        -                                                            h                      2                                        ⁢                                          S                      2                      *                                                        +                                      n                    1                                                                                                                                            r                  2                                =                                                                            h                      1                                        ⁢                                          S                      2                                                        +                                                            h                      2                                        ⁢                                          S                      1                      *                                                        +                                      n                    2                                                                                                          (        1        )            
In the equation, r1, r2 respectively being the received signals of the same space-time encoding module, h1 and h2 respectively being the fading factors of wireless channel between sending antenna and receiving antenna; n1 and n2 being the receiving noises. According to Maximum Likelihood (ML) receiving principle, Signal Noise Ratio (SNR) of judging variable can be computed as:
                              SNR          STTD                =                                            2              ⁢                              σ                0                2                                                    σ              2                                ⁢                      E            s                                              (        2        )            
In the equation, Es being signal transmission power, σ2 being system noise power, σ02 being wireless channel fading character.
In the above method, transmission power is equally divided into two antennas all and singular and the transmission power is not properly distributed between different antennas in terms of certain principle, as a result the best performance is not reached and optimal transmission effect is unattainable.